When you "Navigate to" a waypoint from OpenCPN
, he does the following:
1. Create a temporary route
position to selected point. The route has only two points, start and end.
2. Activate that route.
3. If a suitable output connection is available, he sends the following NMEA
messages periodically to that port.
4. The Cross-track error (XTE) output by OCPN is calculated as for Rhumb Line Sailing, i.e. straight line on Mercator chart.
If an autopilot
, it may choose to follow GC route if it wants to. The current
GPS position and the target waypoint position are available in the messages sent above.
If the A/P uses only XTE, then it is sailing Rhumb Line.
Alternatively, if it ignores XTE, and computes a running GC course based on the target waypoint location and current ownship position, then it is doing GC Sailing.
If the A/P is doing GC Sailing, then you would see the XTE reported by OpenCPN diverging from zero, as expected.
Of course, other chart plotters may do this differently from OpenCPN.
Personally, I wonder why we care. Rare indeed would be the case in which a two-point route long enough to benefit from GC Sailing would be traveled without need to change course for other tactical reasons (weather, traffic, etc.). Could happen with a large motor
vessel, I suppose...
Hope this helps